1. Field of the Invention
The present invention relates to an automatic steering control apparatus for assisting the driver of a motor vehicle in steering the motor vehicle on a road or an expressway to run along a given running path thereon.
2. Description of the Prior Art
There has been proposed in recent years a system wherein path markers such as magnetic nails or the like are installed along a running path on a road and detected by a marker sensor on a motor vehicle to control the motor vehicle to run along the running path while recognizing the positional relationship (representative of a lateral deviation of the motor vehicle from the running path, etc.) of the motor vehicle at a present position from time to time with respect to the running path based on a detected output signal from the marker sensor.
According to the above conventional system, if it is assumed that the steering control quantity (the steering angle) of the motor vehicle is represented by .delta., and the lateral deviation of the motor vehicle at the present position from the running path by .epsilon.y, and the present speed of the motor vehicle by V, then the steering control quantity .delta. is determined by the following equation: EQU .delta.=P(V).multidot..epsilon.y+I(V).multidot..intg..epsilon.ydt
where P(V).multidot..epsilon.y indicates the proportional term of proportional integral control, P(V) indicates a coefficient which is represented by a predetermined function depending on the vehicle speed V, I(V).multidot..intg..epsilon.ydt indicates an integral term, and I(V) indicates a coefficient which is represented by a predetermined function depending on the vehicle speed V. The motor vehicle is steered by an actuator according to the steering control quantity .delta. as is determined by the above equation.
The steering control of the motor vehicle is carried out in a manner to eliminate the lateral deviation .epsilon.y of the motor vehicle from the running path.
With the above steering control process, however, since the steering control quantity is determined depending on only the present speed V of the motor vehicle and the present lateral deviation .epsilon.y of the motor vehicle from the running path, steering control errors tend to be accumulated, causing the motor vehicle to deviate from the running path and to undulate the motor vehicle largely with respect to the running path. Particularly if the motor vehicle runs at a speed of 60 km/h or higher, preferably at a high speed of 100 km/h or higher, then when the motor vehicle is in the vicinity of an entrance of a curve, the ability of the motor vehicle to run along the running path according to the steering control process is reduced, making it difficult to steer the motor vehicle accurately along the running path, or allowing the motor vehicle to deviate into an adjacent lane. In the vicinity of an entrance of a curve, since the lateral deviation .epsilon.y of the motor vehicle from the running path becomes abruptly large, the steering control quantity abruptly changes, resulting in an abrupt change in the steering control process.
To eliminate the foregoing drawbacks, there has been proposed a system for recognizing a road shape such as a road curvature ahead of a motor vehicle with a CCD camera mounted on the motor vehicle, and controlling the steering of the motor vehicle according to the recognized road shape to permit the motor vehicle to run automatically at high speeds.
The proposed system depends on the CCD camera to recognize the road shape ahead of the motor vehicle precisely for accurate steering control. However, when the motor vehicle runs in bad weather such as rain, snow, etc., the imaging capability of the CCD camera is so poor that it is no longer able to accurately recognize the road shape ahead of the motor vehicle. Therefore, it is difficult to steer the motor vehicle under accurate steering control in bad weather such as rain, snow, etc.
One solution would be to use an infrared camera to accurately recognize the road shape ahead of the motor vehicle in a bad weather such as rain, snow, etc. However, the infrared camera is generally expensive and requires a complex camera system. It is not appropriate and practical to install such an infrared camera system on the motor vehicle for accurate steering control.